The Nationals will promote Nate Karns and start him against the Orioles on Tuesday.
Karns will start in place of Ross Detwiler, who has a right oblique strain.
Karns, 25, had pitched this season at Double-A Harrisburg, posting a 4.60 ERA with 11.0 K/9 and 3.6 BB/9. He has not yet pitched at the Triple-A level.